In recent years, internet traffic has been increasing rapidly. To deal with this, techniques for increasing communication capacity have been developed. One of these techniques called wavelength division multiplexing (WDM) increases the capacity by increasing the number of wavelengths.
For the WDM, active efforts have been made to increase the communication capacity excellent in flexibility, in association with development of for example, reconfigurable optical add/drop multiplexers (ROADMs) and optical cross connect (OXC) based on wavelength routing. WDM not only utilizes wavelength resources to increase communication capacity but also to actively utilize wavelength resources to improve network functions.
In an interrelated manner, the increase in communication capacity and the enhancement of functions of a communication scheme enable inexpensive, secure communication services to be provided. When such a communication system is constructed, a wavelength-variable laser light source serves as an important, key device.
In the WDM system according to the related art, a plurality of fixed wavelength light sources with wavelengths spaced at given intervals are arranged. In particular, the cost of backup light sources (the number of which is equal to the number of wavelengths) for maintenance and management is a major factor inhibiting a reduction in system costs.
Applying a wavelength-variable light source to such a WDM system allows light sources of the same type to be used in the entire system. This enables a drastic reduction in system costs. Furthermore, the wavelength-variable light source, which offers a high switching speed, is also an indispensable element for realizing new network functions for wavelength routing.
As a wavelength-variable light source that can cover a C band or an L band, the use of a movable MEMS mirror is described in Non-Patent Document 1 described below. This light source exhibits relatively favorable optical output characteristics. However, there is a concern about the practicality of this light source in terms of production costs and impact resistance.
Furthermore, Non-Parent Document 2 reports a DBR laser (Distributed Bragg Reflector laser) offering improved mode stability and further integrated with a modulator. However, the DBR laser is disadvantageous in terms of costs and reliability.
In connection with a wavelength-variable light source that uses a planar lightwave circuit (PLC) as an external resonator, several configurations have been proposed (for example, Patent Document 1 described below). Furthermore, the PLC is easy to produce and includes no movable section in contrast to MEMS. The PLC is thus excellent in production yield and reliability (particularly impact resistance) and is expected to be suitable for mass production.
Non-Patent Document 3 reports a laser configuration that uses a ring-type external resonator and a semiconductor optical amplifier. The configuration described in this document also includes a wavelength-variable light source with no movable portion but offers favorable characteristics in terms of a wavelength-variable range, an optical output, and the like. However, the thus configured wavelength-variable light source limits the FSR (Free Spectrum Range) of the ring-type external resonator, that is, the wavelength-variable range at the resonance wavelength period.
To increase the wavelength-variable range, it is necessary to reduce the bending radius of a waveguide and thus the size of the ring-type resonator. However, the reduction in the bending radius of the waveguide is limited in connection with optical loss. This is a factor limiting the wavelength-variable range.
Non-Patent Document 1: Jill D. Berger et al., “27th European Conference on Optical Communication (ECOC '01), VOL. 2, 2001, p. 198-199
Non-Patent Document 2: B. Mason et al., “IEEE Photonics Letters”, Vol. 11, No. 6, June 1999, p. 638-639
Non-Patent Document 3: H. Yamazaki et al., “30th European Conference on Optical Communication”, 2004, th4.2.3
Patent Document 1: JP2006-245344A
In a high-density wavelength division multiplexing (D-WDM) transmission scheme in optical communication, the communication wavelength band is divided into a C band and an L band. In each of the wavelength bands, many wavelength signal beams are introduced into a wavelength range of about 40 nm.
Development of many wavelength-variable lasers is now under way based on a wavelength-variable range of 40 nm. Almost no wavelength-variable laser has a large wavelength-variable range (at least 80 nm) such that a single light source can cover both the C band and the L band.
Furthermore, a CWDM (Coarse WDM) scheme in which signal light wavelengths are set at intervals of about 20 nm requires a wider range of wavelength variations.